Perhaps the most important aspect of Instant Runoff Voting (IRV) and Single Transferable Vote (STV) is the ability of the voter to vote for the candidate he or she truly likes, without any fear of "wasting" his or her vote. This is because each voter is allowed to rank the order in which they like the candidates, and each person's vote goes to the highest of their choices who it can help elect. Furthermore, a voter cannot hurt the chances of any candidate he or she prefers by marking lower choices for others.
There are two ways that a vote could be wasted: (1) the candidate that the voter truly likes already has more than enough support to win, or (2) the candidate that the voter truly likes does not have enough support to win. That's why STV does two things: (1) the surplus (or excess) ballots of candidates who already have enough votes to win are each transferred to their next-highest choice, and (2) the ballots of candidates who don't have enough support to win are each transferred to their next-highest choice.
In this country we are still using our antiquated (and defective) 18th century voting methods which allow us to express only one choice on the ballot. Because we are limited to only one choice, we don't dare vote for what we truly want--we vote for what we think we can tolerate.
Ask yourself this: how screwed up is a voting method when a vote for the candidate you like best could end up helping to elect the candidate you like least? Is that right? Is that just? No! There is absolutely no justification for continuing to use our present voting methods (i.e., single-member districts and block voting).
Why do we limit ourselves? Why don't we allow ourselves the right to express our true preferences among ALL the available choices?